The classical apparatus for gamma ray spectroscopy, namely the combination of a crystal scintillation counter with a photomultiplier, has the advantage of having a large detection volume, but the energy resolution still leaves something to be desired. On the other hand, recently developed detectors based on the use of semiconductors in which the gamma quanta are detected in the depopulation zone of a reverse-biased p-n or p-i-n junction or metal-semiconductor contact, have the advantage of providing good energy resolution, although at least in the case of the germanium detector which has so far been nearly exclusively used, the small number of protons in the germanium nucleus (Z = 32) in conjunction with the limited volume are responsible for a detection efficiency which is much inferior to that of a scintillation counter. Moreover, germanium detectors must be cooled to the temperature of liquid nitrogen and the cooling and vacuum system required is a serious obstacle in the way of many applications.
The drawback of the low detection efficiency of semiconductor detectors can be overcome by using semiconductor materials having higher atomic numbers, and compound semiconductors of the type A.sub.II B.sub.VI primarily suggest themselves. Moreover, the generation of thermal carriers of some of these semiconductor materials is so low that operation at room temperature or with only slight cooling is possible (e.g. CdTe, CdS). Although crystals of such semiconductor materials of satisfactory purity have recently been successfully produced, it has nevertheless so far been impossible to overcome the contacting problems which arise and to produce practicable gamma detectors of this type although the principle has been known for a long time (cf. for instance Brit. Pat. Specn No. 995,886). In order to avoid the generation of major dark currents non-injecting contacts must be provided and the high field strengths required for carrier collection, particularly in the case of a less pure semiconductor material, make high demands upon the electrical breakdown resistance of the contacts. The present state of the art does not yet permit pn-junctions which satisfy these several requirements to be produced from these semiconductor materials and the employment of metal-semiconductor contacts is problematic because of local variations in doping and the occurrence of local dielectric breakdown. Consequently the operating voltages and hence energy resolution in gamma detectors using semiconductor materials of high atomic number are in practice governed by the nature of the contacts.